The present invention relates generally to an anti-skid control system for an automotive vehicle with optimal braking characteristics. More particularly, the invention relates to an anti-skid brake control system which can increase braking pressure in discrete steps as well as perform quick take-up of the braking pressure i.e., rapid application of a high braking force, by omitting normal cyclic steps of holding braking pressure at a constant value and/or of decreasing the braking pressure. Proper balance between moderate increase of the braking force by stepwise braking pressure build-up and rapid increase by quick take-up ensures optimal braking characteristics under all road surface conditions.
In the prior art, it has been considered that optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. If the wheel speed/vehicle speed ratio is maintained higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to a lack of braking pressure. On the other hand, if the braking pressure is so adjusted as to maintain the wheel speed/vehicle speed ratio less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed.
In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain at an optimal relationship to the vehicle speed for a satisfactorily long period of time.
U.S. Pat. No. 4,384,330, granted to the assignee of the present invention, discloses a brake control system for controlling application and release of brake pressure in order to prevent the vehicle from skidding. The system includes a sensing circuit for determining wheel rotation speed, a deceleration detecting circuit for determining the deceleration rate of the wheel and generating a signal when the determined deceleration rate becomes equal to or greater than a predetermined value, a target wheel speed circuit for determining a target wheel speed based on the wheel rotation speed and operative in response to detection of a peak in the coefficient of friction between the vehicle wheel and the road surface, and a control circuit for controlling application and release of brake fluid pressure to wheel cylinders for controlling the wheel deceleration rate. The wheel rotation speed sensing circuit detects the angular velocity of the wheel to produce alternating current sensor signal having a frequency corresponding to the wheel rotation speed. The wheel rotation speed sensor signal value is differentiated to derive the deceleration rate.
In another approach, U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.
In addition, Published Japanese Patent Application (Tokkai) No. Showa 51-89096, published on Aug. 4, 1976 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.
In order to improve the foregoing prior art, there has been proposed an improved anti-skid brake control system in the co-pending U.S. patent application Ser. No. 634,489, filed in July 26, 1984, also pending before European Patent Office under Application No. 84108356.1, filed on July 16, 1984. In the above-identified prior proposal, moderate increase of the braking pressure has been improved by skipping cyclically executed steps of holding the braking pressure at a constant value when the road surface condition provide relatively high road/tire friction so as to allow wheel speed to recover at a rate higher than a predetermined value.
Another approach has been proposed in the copending U.S. patent application Ser. No. 673,200, filed on Nov. 19, 1984, corresponding to European Patent Application No. 84113434.9, filed on Nov. 7, 1984. In the disclosed system, braking pressure holding and/or decreasing is disabled to allow continuous increase of the braking pressure toward a lock pressure, when relatively high road/tire traction conditions are detected.
The present invention is intended to further improve the aforementioned proposals of the common applicant and so optimize braking characteristics not only under relatively high road/tire traction conditions but also at relatively low traction.